1. Field
Disclosed herein is a method for recovering gold in connection with the hydrometallurgical production of copper from a waste or intermediate product containing sulphur and iron that is generated in the leaching of the copper raw material. The recovery of both copper and gold occurs in a chloride environment. The gold contained in the waste or intermediate is leached by means of divalent copper, oxygen and bromide in a solution of copper (II) chloride-sodium chloride, in conditions where the oxygen-reduction potential is a maximum of 650 mV and the pH is between 0.5-2.5.
2. Description of Related Art
The Hydrocopper™ method for fabricating copper hydrometallurgically from a raw material containing copper such as copper sulphide concentrate is described in U.S. Pat. No. 6,007,600. According to the method, the raw material is leached counter-currently with an alkali chloride-copper chloride solution in several stages to form a monovalent copper (I) chloride solution. Part of the solution that is formed is routed to oxidation performed with chlorine gas, whereupon the copper (II) chloride generated is circulated back to concentrate leaching. Solution purification is performed on the remainder of the solution formed in raw material leaching. The pure cuprous chloride solution is precipitated by means of alkali hydroxide into copper (I) oxide and the oxide is reduced further into elemental copper. The alkali chloride solution formed during copper (I) oxide precipitation is processed further in chlorine-alkali electrolysis, from which the chlorine gas and/or chloride solution obtained is used for oxidising copper (I) chloride and/or raw material leaching, the alkali hydroxide generated in electrolysis is used for oxide precipitation and the hydrogen generated is used for elemental copper reduction. Gold recovery from the leaching residue is not described separately in connection with the method.
Several methods are known in the prior art, which are used for leaching gold from materials containing sulphur and iron in connection with a chloride-based copper recovery process.
U.S. Pat. No. 4,551,213 describes a method, according to which gold can be leached from sulphur-containing materials, particularly from the residues of hydrometallurgical processes. The preferred starting material for the method is residue from the CLEAR process. The CLEAR process is a hydrometallurgical copper recovery process, which takes place in a chloride environment and at raised pressure. The gold-containing residue is elutriated into water and the suspension obtained is adjusted so that it contains 12-38 weight percent of chloride. The oxidation-reduction potential is adjusted to the range of 650-750 mV and the pH value to be below 0. Copper (II) chloride or iron (III) chloride is added to the suspension to oxidise the gold contained in the raw material, whereupon it dissolves. It is mentioned in the publication that the oxidation-reduction potential must not rise above 750 mV, because above this value the sulphur will dissolve. There is no information in the publication about the amount of dissolved sulphur or iron.
EP patent 646185 concerns the recovery of copper from sulphidic concentrates by chloride leaching in atmospheric conditions. In the final stage of countercurrent leaching, gold is leached directly into the electrolyte from copper, zinc and lead electrolysis with a high oxidation potential. It is an essential feature of the method that the high oxidation potential is achieved by means of a halide complex such as BrCl−2, which is formed in electrolysis. According to example 4, which describes gold leaching, gold dissolves at an oxidation-reduction potential of about 700 mV vs Ag/AgCl.
WO patent application 03/091463 describes a method for leaching gold from a leaching residue or intermediate containing iron and sulphur, which is generated in the atmospheric chloride leaching of copper sulphide concentrate. It states in the publication that it is possible to leach gold from a material containing iron and sulphur into an aqueous solution of copper (II) chloride and sodium chloride by means of divalent copper and oxygen in conditions where the oxidation-reduction potential is below 650 mV and the pH value of the solution is in the range of 1-3. In these conditions the iron does not yet dissolve and the sulphur remains largely undissolved, thus avoiding the costs that are incurred when removing iron and sulphur from the solution. The recovery of gold from the solution is carried out by means of one of the methods of the prior art such as electrolysis or activated carbon. The method in question is fairly good in itself, but in practice however it is a little slow.